This would be A) you double clicked.Just copy pasta to your addons folder.B)You have a L4D2 mod for the Special infected or Common infected still in your L4D1 addons folder.Left 4 Dead is not backwards compatable I'm sorry to say.Go to Program files (X86)/steam/steam apps/common/left 4 dead/left 4 dead/addons.Delete all L4D2 addons.